1. Field of the Invention
The present invention relates to a constant temperature crystal oscillator (hereinafter referred to as a constant temperature oscillator) using a surface-mount crystal resonator (hereinafter referred to as a surface-mount resonator), and more specifically to a constant temperature oscillator which excels in response characteristic to a temperature change.
2. Description of the Related Art
Generally, a constant temperature oven has been used for a constant temperature oscillator. Since the operation temperature of a crystal resonator can be kept constant, the frequency stability is high (the frequency deviation is approximately 0.05 ppm or lower). For example, it is used for the communication facilities of a base station for optical communications, etc. Recently, these communication facilities have become downsized. In this connection, a surface-mount resonator has been widely adopted. The Applicant of the present invention has disclosed one of these facilities (Japanese Patent Application No. 2004-157072).
FIGS. 1A, 1B, 2A, and 2B are explanatory views showing a related art. FIG. 1A is a sectional view of a constant temperature oscillator. FIG. 1B is a plan view of the first substrate. FIG. 2A is a sectional view of a surface-mount resonator. FIG. 2B is a bottom view.
The constant temperature oscillator has a surface-mount resonator 1A, an oscillation circuit element 1 for forming an oscillation circuit together with the resonator, and a temperature control element 2 for keeping a constant operation temperature of the surface-mount resonator 1A arranged on a circuit substrate 3, and these components are airtightly sealed in a metal container 4. The surface-mount resonator 1A fixes a crystal element 7 using a conductive adhesive 6 on the inside bottom portion of a concave ceramic container body 5, a metal cover 8 is used as a cover and airtightly seals the entire structure.
At the four corners of the outside bottom portion (reverse side) of the container body 5, crystal terminals 9a and dummy terminals 9b are provided as mount terminals for a set substrate of a wireless equipment, etc. The crystal terminals 9a (two terminals) are provided at a set of diagonal portions, and connected to an excitation electrode (not shown in the attached drawings) of the crystal element. The dummy terminals 9b (two terminals) are provided at the other diagonal portions, and are normally connected to the metal cover 8 using a via hole, etc. (not shown in the attached drawings), and function as, for example, grounding terminals connected to a grounding pattern (not shown in the attached drawings) of the substrate.
The temperature control element 2 keeps the constant operation temperature of the surface-mount resonator 1A, and includes at least a heating chip resistor 2a (for example, two resistors), a power transistor 2b for supplying power to the resistors, and a temperature sensitive resistor 2c for detecting the operation temperature of the surface-mount resonator 1A. The temperature sensitive resistor 2c is assumed to be a thermistor indicating a decreasing resistance value with an increasing temperature. The power transistor 2b provides the power controlled by the resistance value based on the temperature of the temperature sensitive resistor 2c for the heating chip resistor 2a. Thus, the operation temperature of the surface-mount resonator 1A is kept constant.
The circuit substrate circuit substrate 3 includes a first substrate 3a and a second substrate 3b, and the second substrate 3b is held by a metal pin 10a on the first substrate 3a. The first substrate 3a is made of a glass epoxy material, and the oscillation circuit element 1 excluding the surface-mount resonator 1A is arranged on the bottom surface. The second substrate 3b is made of a ceramic material, and has the crystal resonator 1A arranged on the top surface, and has the chip resistor 2a and the temperature sensitive resistor 2c excluding the power transistor 2b in the temperature control element 2 arranged on the bottom surface.
Between the first substrate 3a and the second substrate 3b, a silicon base thermal conductive resin 11 is applied for covering the chip resistor 2a and the temperature sensitive resistor 2c. Since the power transistor 2b is long in height, it is arranged on the terminal side of the first substrate 3a. The metal container 4 is formed by a metal base 4a and a cover 4b. An airtight terminal 10b of the metal base 4a holds the first substrate 3a, and the cover 4b airtightly sealed by resistance welding. The dummy terminal 9b as a grounding terminal of the crystal resonator 1A is connected to the airtight terminal 10b for grounding through the conductive path (grounding pattern) not shown in the attached drawings and the metal pin 10a. 
In this example, electric power is supplied to the heating chip resistor 2a by, for example, a well-known temperature control circuit shown in FIG. 3A. That is, a temperature sensitive voltage by the temperature sensitive resistor 2c and a resistor Ra is applied to one input terminal of an operational amplifier 12, and a reference voltage by resistors Rb and Rc is applied to the other input terminal. Then, the reference temperature difference voltage from the reference voltage is applied to the base of the power transistor 2b, and electric power is supplied from the direct current voltage DC to the heating chip resistor 2a. Thus, the electric power to the heating chip resistor 2a can be controlled by the resistance value depending on the temperature of the temperature sensitive resistor 2c. 
Normally, before connecting the metal cover 4b by setting the first and second circuit substrates 3a and 3b to the metal base 4a, for example, the frequency temperature characteristic as the cubic curve shown in FIG. 3B of the surface-mount resonator 1A as, for example, AT cut is individually measured. When the temperature as the minimum value at the high temperature side of the operation temperature of the surface-mount resonator 1A is, for example, 80° C., the resistor Ra of the temperature control circuit is controlled and the surface-mount resonator 1A is set to 80° C. Then, the control capacitor (not shown in the attached drawings) of the oscillation circuit matches the oscillation frequency f with the nominal frequency. Thus, the resistor Ra and the control elements 13 which require exchange such as the control capacitor, etc. are arranged on the perimeter of the first substrate 3a horizontally projecting from the second substrate 3b (FIG. 1).
With the above-mentioned configuration, the conventional constant temperature oven not shown in the attached drawings is not used, and the heating chip resistor 2a is used as a heat source. Therefore, the entire system can be basically downsized. Then, the second substrate 3b having the surface-mount resonator 1A, the chip resistor 2a, and the temperature sensitive resistor 2c is a highly thermal conductive ceramic material. These components are covered with a thermal conductive resin. Therefore, the operation temperature of the surface-mount resonator 1A can be directly detected by the temperature sensitive resistor 2c, and the response characteristic to a temperature change can be improved.
However, in the constant temperature oscillator with the above-mentioned configuration, although the surface-mount resonator 1A and the temperature sensitive resistor 2c are arranged on both main surface sides of the second substrate 3b which is made of a highly thermal conductive ceramic material, the thermal conductivity is low (poor thermal conductivity) as compared with copper (Cu), gold (Au), etc. as a wiring pattern, for example. Therefore, since the resistance value of the temperature sensitive resistor is not immediately changed in synchronization with the operation temperature of the surface-mount resonator, and the operation temperature cannot be detected in real time, there has been the problem that the response characteristic to an ambient temperature is poor.
Since the first substrate 3a and the second substrate 3b are arranged up and down by the metal pin 10a, the number of production processes can be increased and the height is increased. Furthermore, since the second substrate 3b is made of a ceramic material, it is more expensive than a substrate of a glass epoxy material. In addition, since the power transistor 2b of the temperature control element 2 is long in height, it is arranged on the first substrate 3a aside from the second substrate 3b on which the chip resistor 2a is arranged. Therefore, the liberated heat from the power transistor 2b can be prevented from being effectively used.